Such a position-measuring device for determining the relative position of two objects movable relative to each other (by a rotating motion) along a measuring direction (an angular position measuring device) includes: a periodic measuring graduation configured on a ring-like measuring standard device, e.g., in the form of a measuring strip; a scanning unit for optically scanning the measuring graduation using electromagnetic radiation (e.g., using light) which is generated in an illumination device (light source) of the scanning unit, the measuring standard device and the scanning unit being movable relative to each other along the measuring direction; a scanning plate of the scanning unit having a scanning graduation extending along an axis, that is formed by a scanning grating, which is arranged in the beam path of the electromagnetic radiation used for scanning the measuring graduation, so that this electromagnetic radiation interacts both with the scanning graduation and with the measuring graduation; and a detector of the scanning unit, the detector surface of which is used for detecting an intensity distribution of the electromagnetic radiation that is present as a stripe pattern, after interaction with the scanning graduation and the measuring graduation, so as to be able to record a relative motion of the measuring graduation with respect to the scanning device, from the change of the position-dependent phase of the intensity distribution.
By furnishing one of the two objects, whose relative motion is to be determined using the position measuring device, with the measuring graduation, and the other object with the scanning device, from the recording of the relative motion of the scanning device with respect to the measuring graduation, one may directly draw conclusions on the relative motion of the two objects to each other.
The measuring principle on which the aforesaid position-measuring device is based is described, for example, in European Published Patent Application No. 1 081 457, European Published Patent Application No. 1 028 309 and European Published Patent Application No. 0 754 933. European Published Patent Application No. 1 028 309 describes an optical position-measuring device for determining the relative position of two objects movable relative to each other along a measuring direction. This includes at least one periodic measuring graduation connected to one of the two objects, as well as a scanning unit connected to the other object and therefore movable relative to the measuring graduation along the measuring direction. The scanning unit has a light source, at least one scanning graduation, as well as a detector system in a detector plane that is made up of a plurality of radiation-sensitive detectors arranged one after the other in the measuring direction.
If, in a position-measuring device, a measuring strip used as the carrier of the measuring graduation is arranged in ring-like fashion, particularly by applying the measuring strip on a cylindrical body in the form of a drum, the position-measuring device may then be used for angular measurement. Alternatively, the respective measuring graduation may, for this purpose, also be applied directly on the drum itself, so that then the drum surface furnished with the measuring graduation directly forms the ring-like circumferential measuring standard device.
In the following, when using the term “measuring strip,” provided it is not geared to a measuring strip of finite thickness, the intention is in each case also to include the limiting case in which the thickness of the measuring strip tends to zero: This limiting case corresponds to a situation in which the measuring graduation is applied directly on the (e.g., cylindrical) body (e.g., in the form of a drum), which is otherwise used for accommodating the measuring strip.
Using such an arrangement, there may be the problem that the local graduation period, projected onto the plane of the scanning plate, of the periodic measuring graduation provided on the ring-like circumferential measuring standard device decreases toward the outside with growing distance from the apex of the drum. From this there comes about a mutual shifting, growing toward the outside, of the graduation marks of the scanning graduation on the one hand, and of the measuring graduation on the other hand, with regard to their optimal position that would occur in response to a linear periodic arrangement both of the measuring graduation and of the scanning graduation. The described deviations, that follow from the ring-like arrangement of the periodic measuring graduation, may lead to a deterioration in the degree of modulation of the scanning signal generated at the detector.